Effect Of Rare Earth Ce And Heat Treatment On Inclusions And Mechanical Properties In U75V Steel

In this thesis,we successfully smelt U75V heavy rail steel with national standard composition and U75V steel containing 8 ppm,32 ppm and 42 ppm rare-earth Ce using vacuum induction furnace as the research object,and studied the effects of rare-earth Ce content and heat treatment system on inclusions and mechanical properties in the experimental steel.The research process includes literature research,thermodynamic calculations,dosing calculations,smelting of experimental steels,composition testing,experimental steel tying,heat treatment,inclusions analysis,microstructure analysis,hardness analysis,impact property analysis,tensile property analysis and wear property analysis.Through thermodynamic calculations,it can be found that the inclusions in the original U75V heavy rail steel without the addition of rare earth Ce are mainly MnS and Al₂O₃,and the inclusions in the steel after the addition of rare earth Ce are mainly composite inclusions of MnS,Al₂O₃and SiO₂and Ce₂O₂S,because the combination ability of rare earth Ce and sulfur is stronger than that of Mn and sulfur,so the content of MnS phase in the experimental steel decreases with the increase of rare earth content,and the content of Ce₂O₂S phase increases with the increase of rare earth content.The content of MnS phase in the experimental steel decreases continuously with the increase of rare earth content,and the content of Ce₂O₂S phase increases constantly with the increase of rare earth content.After observing the morphology and composition of the inclusions in the experimental steel using scanning electron microscopy and energy spectrometry,it was found that the addition of rare earth Ce to the steel,the rare earth elements play a role in modifying the inclusions in the steel,which helps to reduce the influence of the inclusions on the continuity and stress concentration of the steel matrix is beneficial to the improvement of the mechanical properties of the steel.The size and aspect ratio of inclusions were calculated by using Image-J software,and it was found that the size and aspect ratio of inclusions in steel with 42 ppm rare earth Ce added and heated to 1200℃ for 2 h water cooling were most significantly reduced compared to U75V heavy rail steel without rare earth elements or with single rare earth elements added without heat treatment,the average inclusions size was reduced by about 75%and the average inclusions aspect ratio was reduced by about 55.4%compared to the original U75V steel,helps reduce the impact of large,irregular inclusions on steel properties.The pearlite lamellae spacing in U75V heavy rail steel without and with different contents of rare earth Ce were measured and averaged by using Image-J software,and it was found that the pearlite lamellae spacing of the experimental steel decreased continuously with the increase of rare earth content compared with the No.1 steel without the addition of rare earth Ce.The pearlite layer spacing of No.4 steel with 42 ppm rare earth Ce was reduced by about 75%compared with the original steel.The grain size of the experimental steel after heat treatment was measured by Image-J software and averaged,heat treatment was carried out singly,and grain size increased significantly in No.1 steel with increasing holding time and the results showed that the grain size of U75V steel decreased with the increase of rare-earth content compared with steel No.1 without the addition of rare-earth Ce,and the grain size of steel No.4with the addition of 42 ppm rare-earth Ce was the smallest and decreased by about 35%compared with the original steel.The microhardness analysis of the experimental steel shows that,without heat treatment,the hardness of No.2 steel with 8 ppm rare earth Ce and No.4 steel with 42 ppm rare earth Ce increase by about 0.8%and 2.6%,respectively,compared with No.1 steel without rare earth Ce.The hardness of No.1 steel without rare earth is 35.7%higher than that of No.1 steel without heat treatment after being held at 1200℃ for 1 h and then cooled by water.Compared with the original U75V steel,the hardness of No.2 U75V steel with 8 ppm rare earth Ce is improved by about 49.5%after being heated by 1200℃ and held for 1 h by water cooling.Compared with No.1 steel,the impact energy of No.2 steel with 8 ppm rare earth Ce is increased by 61%,29%and 85%at 20℃,-20℃ and-60℃,respectively.At-40℃,the impact energy of No.3 steel with 32 ppm rare earth Ce increases by about 98%compared with that of No.1 steel without Ce.The tensile properties of the steel were analyzed and it was found that the yield strength,tensile strength and elongation of the experimental steel No.4 with the addition of 42 ppm rare earth Ce showed the most significant increase of 18.5%,12.9%and 8.1%,respectively,compared to the original U75V steel without the addition of rare earth Ce.Analysis of the wear experimental data showed that the wear amount of U75V steel with the addition of rare earths was reduced by about 50%compared to the U75V steel without the addition of rare earths.As for the wear coefficient,the smaller the average friction coefficient of the material,the better the wear resistance of the material.The average wear coefficient of the No.4 steel with 42 ppm rare earth Ce addition is 0.6792,which is about 5.7%less compared with the No.1 steel without rare earth Ce addition and has the best wear resistance.